**Question:** An enzyme that makes a double-stranded DNA copy from a single-stranded RNA template molecule is known as

A. RNA polymerase
B. DNA polymerase
C. DNA ligase
D. DNA primase

**Correct Answer:** C. DNA ligase

**Core Concept:** Enzymes play a crucial role in the process of gene expression where they catalyze specific chemical reactions in cells. In the context of this question, we are discussing an enzyme involved in synthesizing DNA from RNA templates. This process is called transcription, where RNA polymerase reads the DNA template and synthesizes RNA complementary to the template strand. To complete the process, DNA ligase is essential.

**Why the Correct Answer is Right:** DNA ligase is a crucial enzyme involved in the final step of DNA replication, which is the sealing of the nicked DNA strands after the DNA polymerase has synthesized the new strands. DNA ligase directly joins the newly synthesized DNA strands together by forming phosphodiester bonds between the 3'OH and 5'phosphate ends of the DNA strands. This ensures the stability and completeness of the DNA molecule.

**Why Each Wrong Option is Incorrect:**

A. RNA polymerase is responsible for synthesizing RNA from DNA templates during transcription, not DNA from RNA templates.
B. DNA polymerase is an essential enzyme in DNA replication, but it synthesizes DNA strands complementary to the template DNA strand, not from RNA templates.
D. DNA primase synthesizes short RNA primers during DNA replication, not synthesize DNA from RNA templates.

**Clinical Pearl:** Understanding the roles of these key enzymes helps medical students and practitioners grasp the complexity of cellular processes and their implications in various clinical scenarios, such as genetic disorders, mutations, and viral infections. For instance, defects in DNA ligase can lead to genomic instability and genomic rearrangements, causing conditions like ataxia-telangiectasia.